1. Field of the Invention
The present invention relates to a method of evaluating a spot-type ionizer in which a driving voltage is applied to discharge needles and ion air containing plus ions and minus ions generated through corona discharge is blown in a spot manner from a nozzle opening onto a target to neutralize static electricity and, particularly, to a spot-type ionizer evaluation method and spot-type ionizer in which, for an ion balance and a decrease in ion balance variation, a grid is attached to the nozzle opening to generate an optimum use condition.
2. Description of the Related Arts
Conventionally, in a hard disk drive manufacturing process, a semiconductor manufacturing process, a liquid-crystal manufacturing process, and the like, an ionizer is used in order to prevent an electrostatic hazard in a clean room. The ionizer functioning as a static eliminator performs static elimination by neutralizing static electricity, which would cause a trouble, such as product destruction or an erroneous operation of equipment. Depending on the method of generating ions, ionizers are divided into those of AC scheme and those DC scheme. An AC ionizer applies an alternate-current voltage to a discharge needle for corona discharge to alternately generate plus ions and minus ions. Also, a DC ionizer applies a direct-current voltage to a pair of discharge needles for corona discharge to generate plus ions from a plus-side discharge needle and simultaneously minus ions from a minus-side discharge needle. Also, ionizers include those of a distribution type for distributing the generated plus ions and minus ions to a wider area (JP2000-100596 and JP2003-28472) and those of a spot type for blowing the generated plus ions and minus ions onto the target in a spot manner with compressed air. In a hard disk drive manufacturing process, a semiconductor manufacturing process, a liquid-crystal manufacturing process, and the like, fine device targets are targets for static elimination. Therefore, a spot-type ionizer with a small ion balance and a shorter static elimination time are used. As indicators for evaluating the performance of the ionizer, an ion balance and a static elimination time have been known, which are measured with the use of a charge-plate monitoring device. The charge-plate monitoring device is configured of a measurement plate and measuring unit body, in which the potential of the measurement plate is measured by the measuring unit body and can be on digital display. Here, the ion balance represents a value obtained by, after connecting the measurement plate to the ground and setting an indication of a plate voltage to 0 V, blowing ion air of the ionizer onto the measurement plate and measuring a plate potential. At this time, if the plus ions and the minus ions generated by the ionizer are equal to each other, the ion balance (plate voltage) is stable near 0 V. That is, if the ion balance is stable near 0 V, it can be said that the performance of the ionizer is high. Also, the static elimination time is a time taken from the time when the voltage of the measurement plate is increased to, for example, 1000 V, to the time when ion air from the ionizer is applied onto the measurement plate until the voltage of the measurement plate is attenuated to, for example, 100 V. Similarly, it can be said that, as the static elimination time is shorter, the performance of the ionizer is higher. Generally speaking, in a distribution-type ionizer for wide-area static elimination, the ion balance does not pose much problems because the target has a high withstanding voltage. However, in a spot-type ionizer for use in a hard disk drive manufacturing process, a semiconductor manufacturing process, a liquid-crystal manufacturing process, and the like, the ion balance has to be decreased as much as possible because the target has a low withstanding voltage. In particular, with the increase in integration and response speed of semiconductors in recent years, the withstanding voltage of electronic devices with respect to static electricity is decreasing. In a semiconductor manufacturing process, an ion balance of ±5 to ±10 V is required. Furthermore, in a hard disk manufacturing process, a further lower ion balance equal to or lower than ±5 V or ±3 V or further equal to or lower than ±1 V is required. Here, in some conventional distribution-type ionizers disclosed in the references 1 and 2, a grid using a metal net is placed at an entrance or exit of the ion air. However, a distance from the exit of the ion air to the target is often equal to or longer than 1 meter, which is considerably distant. With such a distance, due to an ionic bond with ions exiting in the air, the ion balance is stabilized. Therefore, a change in ion balance cannot particularly been observed between the case where a grid is placed and the case where no grid is placed.
However, in the conventional spot-type DC ionizer, at the charge-plate monitoring device, after zero-point adjustment in which the measurement plate is connected to the ground and an indication of a plate voltage is set to 0 V, ion air of the ionizer is blown onto the measurement plate, and a plate potential is measured, thereby checking an ion balance at 0 V from the display value of a digital voltmeter. In such a case where, after the ion balance is checked, the target device is processed while the ion air is blown on to the target device, even an ion balance is achieved, an electrostatic breakdown of the target device occurs at some frequencies, posing a problem of not always ensuring the performance of the ionizer. Moreover, in a spot-type AC ionizer, with a known normal use distance on the order of 5 to 10 cm, the ion balance is increased to be equal to or larger than 20 V. To decrease the ion balance within ±1 V, the ionizer has to be set with a use distance equal to or longer than 30 cm. However, if the use distance of the AC ionizer is as much as 30 cm, the ion air is diffused, the ionizer ceases to function as a spot-type, the static elimination time is significantly increased to exceed use limitations. To get around this, the air pressure supplied to the AC ionizer is significantly increased to, for example, 1.0 MPa to ensure a sufficiently short static elimination time. However, if the air pressure is increased in such a manner, a large noise occurs due to compressed air jetted from a nozzle opening of the AC ionizer, thereby significantly increasing a noise level in a working environment.